It is known to use catalytic combustion in combustion turbine engines to reduce NOx emissions. One such catalytic combustion technique known as lean catalytic, lean burn (LCL) combustion, involves completely mixing fuel and air to form a lean fuel mixture that is passed over a catalytically active surface prior to introduction into a downstream combustion zone. However, the LCL technique requires precise control of fuel and air volumes and may require the use a complex preburner to bring the fuel/air mixture to lightoff conditions. An alternative catalytic combustion technique is the rich catalytic, lean burn (RCL) combustion process that includes mixing fuel with a first portion of air to form a rich fuel mixture. The rich fuel mixture is passed over a catalytic surface and mixed with a second portion of air in a downstream combustion zone to complete the combustion process. U.S. Pat. No. 6,415,608 describes a gas turbine engine having an annular combustor design using catalytic reactor elements in an RCL configuration. The catalytic reaction takes place in a series of annularly mounted modules, each module comprising a catalytic reactor element, a fuel injection region, a rich fuel/air mixing region, and a downstream mixing zone at the catalytic reactor element exit.
The design of a turbine engine combustor is further complicated by the necessity for the engine to operate reliably with a low level of emissions at a variety of power levels. High power operation at high firing temperatures tends to increase the generation of oxides of nitrogen. Low power operation at lower combustion temperatures tends to increase the generation of carbon monoxide and unburned hydrocarbons due to incomplete combustion of the fuel. Under all operating conditions, it is important to ensure the stability of the flame to avoid unexpected flameout, damaging levels of acoustic vibration, and damaging flashback of the flame from the combustion chamber into the fuel premix section of the combustor. A relatively rich fuel/air mixture will improve the stability of the combustion process, but will have an adverse affect on the level of NOx emissions. A careful balance must be achieved among these various constraints in order to provide a reliable engine capable of satisfying very strict modem emissions regulations. A pilot flame is commonly used to stabilize the flame during engine loading conditions. However, pilot nozzles may produce a significant portion of the NOx produced by the combustion engine. In addition, the mechanical intricacy of a pilot flame nozzle and fueling of the pilot flame introduce undesirable expense and complexity to the combustor.
Staging is the delivery of fuel to the combustion chamber through at least two separately controllable fuel supply systems or stages including separate fuel nozzles or sets of fuel nozzles. Staging is known as a method to control combustion under varying loading conditions. As the power level of the machine is increased, the number of stages brought on-line is increased to achieve a desired power level. A two-stage can annular combustor is described in U.S. Pat. No. 4,265,085. The combustor of the '085 patent includes a primary stage delivering fuel to a central region of the combustion chamber and a secondary stage delivering fuel to an annular region of the combustion chamber surrounding the central region. However, a centrally located pilot is still required in the combustor of the '085 patent, resulting in undesirable NOx production.
Accordingly, there is a need for improved control of combustion in gas turbine engines to reduce NOx formation.